This invention pertains to a simple, inexpensive, anaerobic digester that efficiently and quickly digests primarily organic aqueous and sludge-type wastes using a plug-flow system comprising a series of sequential reaction chambers.
Various designs of digesters exist for the processing and treatment of primarily organic wastes (solids, semi-solids, and liquids) to produce non-hazardous, and sometimes beneficial, products for release to the environment. Digesters may be designed for use in low technology rural areas or for sophisticated industrial areas. Many types of organic wastes (i.e., municipal, industrial, agricultural, and domestic wastes) maybe treated by anaerobic digestion. See F. R. Hawkes et al., xe2x80x9cChapter 12: Anaerobic Digestion,xe2x80x9d in Basic Biotechnology (J. Bu""Lock and B. Kristiansen, eds.) pp. 337-358, (Academic Press, Orlando, Fla., 1987).
Most digesters are based on either aerobic or anaerobic fermentation, although some combine elements of both. The objectives of all such digestion processes are to reduce the total amount of sludge solids, and to produce a cleaner effluent for discharge to the environment or for further processing prior to discharge. Successful anaerobic digestion of organic wastes usually requires a mixed culture of bacteria with a complex interdependency, terminating in the production of methane by methanogenic bacteria. Hawkes et al., 1987. Waste digesters that use anaerobic processes have at least two advantages over those that use aerobic digestion: (1) anaerobic digestion produces methane, which can be used as a fuel gas either internally or sold commercially; and (2) anaerobic digestion is generally more efficient at removing solids, and thus produces less sludge than aerobic digestion. See U.S. Pat. No. 4,885,094.
The main disadvantage of anaerobic digesters is the long residence time typically required to digest organic waste. Many anaerobic digesters are xe2x80x9cbatchxe2x80x9d or one-stage digesters, e.g., comprising a closed or domed vessel within which very large quantities of organic waste are fermented in batch. Anaerobic batch digesters can take 20 to 30 days to adequately digest the organic solids. See U.S. Pat. No. 5,637,219. Although these batch digesters can handle large quantities of waste, the prolonged time usually required for digestion has limited their use for municipal or industrial waste. As a result, many municipal and industrial wastes are processed using aerobic digestion systems or a combination of aerobic with anaerobic systems. See U.S. Pat. No. 4,885,094.
The microbiology of anaerobic digestion can be generally described as comprising four broad trophic groups, which digest organic materials in sequence. The first group, the hydrolytic and fermentative bacteria, contains both obligate and facultative anaerobes, and removes small amounts of oxygen that may be introduced into the digester with the waste influent. By hydrolysis, this group initially breaks down the more complex molecules (e.g., cellulosics, starch, proteins, lipids, etc.) into smaller units (e.g., amino acids, sugars, and fatty acids). Then, by a process of acidification, this group uses these smaller compounds to produce formate, acetate, propionate, butyrate, hydrogen, and carbon dioxide. These acidic products are then available for the next trophic level. In many digesters, the rate-limiting step is the hydrolysis of complex molecules, particularly the polysaccharides. See F. R. Hawkes et al., 1987.
The second trophic group comprises hydrogen-producing acetogenic bacteria, or proton-reducing bacteria. By a process of acetification (also called xe2x80x9cacidificationxe2x80x9d), this group makes acetate from compounds such as fatty acids, butyrate, formate, and propionate.
The third trophic group of bacteria, comprising homoacetogenic bacteria, produces acetate from hydrogen gas and carbon dioxide. The significance of this group in digester operation is uncertain.
The final trophic group comprises the methanogenic bacteria, which convert compounds such as acetate into methane gas and carbon dioxide in a process called methanogenesis. This group is strictly anaerobic, requiring an oxygen-free environment.
Two important limitations of digesters are the rate at which waste can be processed, and the fraction of solids in the waste that can be digested. The loading rate or flow rate determines the residence time in the digester. The residence time required by standard-rate anaerobic digesters whose contents are unmixed and unheated for the microorganisms to produce a clean effluent is quite long on the order of 30 to 60 days. Optimum anaerobic performance is achieved by proper mixing and heating. Mixing has been achieved by gas injection, mechanical stirring, and mechanical pumping. High-rate digesters whose contents are both heated and mixed have an effective residence time of about 4 days to 15 days, depending on the temperature. The shortest residence time of 4 days was for a temperature of 40xc2x0 C. See Metcalf and Eddy, Inc., Wastewater Engineering, 3rd Edition, revised by G. Tchobanoglous and F. L. Burton (1991), especially Chapter 8: xe2x80x9cBiological Unit Processes,xe2x80x9d pp. 359-444; and Chapter 12: xe2x80x9cDesign of Facilities for the Treatment and Disposal of Sludge,xe2x80x9d pp. 765-926.
Wastes are often characterized by the fraction of solids in the waste. One arbitrary classification scheme is low, medium, and high strength wastes, and solid wastes. These four categories can be divided on the basis on dry matter or total solids (TSS) content as corresponding roughly to 0.2-1%, 1-5%, 5-12%, and 20-40% solids by weight, respectively. TSS is also expressed as mg/L, where 20,000 mg/L equals 2% solids. TSS includes both inorganic and organic solids. To measure only organic matter, either a determination of volatile solids is made by combusting all the organic material, or the organic material is chemically oxidized to give a measurement of Chemical Oxygen Demand (COD). See F. R. Hawkes et al., 1987.
Anaerobic digesters include both batch and continuous digesters. A continuous process is usually favored, since the waste is processed continuously, and there is a steady supply of methane. The classic design for industrial digesters is a variant of a one-stage digester, the continuously stirred tank reactor (xe2x80x9cCSTRxe2x80x9d). In a CSTR digester, all contents are completely mixed. Thus the effluent will contain some amount of freshly added, undigested waste material, and will include some active microbes. The CSTR is usually used for waste with a medium solids content, from 2 to 10% dry matter. Two alternative designs to overcome these problems are the xe2x80x9cplug-flowxe2x80x9d digester and the microbe retention digester. In a plug-flow digester, the waste passes through the digester in a sequential manner from the inlet to the outlet. The name xe2x80x9cplug-flowxe2x80x9d is usually used for designs that are unstirred and tubular. The solid material tends to move through the digester sequentially, while the liquid fraction mixes more rapidly. The retention digester is designed to retain the microorganisms in the digester. The most successful design is based on the upflow anaerobic sludge blanket (UASB), in which the waste enters the base of the digester and flows upwards through a sludge of settled bacteria. The treated waste emerges at the top and passes into a zone where any bacteria in the effluent can settle out back into the digester. However, the UASB is only useful with wastes containing low amounts of solids, typically less than 1%. See F. R. Hawkes et al., 1987.
Some anaerobic digesters are considered two-stage digesters, because the processes of hydrolysis and acidification are separated from the processes of acetification and methanogenesis. This separation usually produces methane gas with lower levels of impurities. See U.S. Pat. No. 5,637,219. Complex, multi-stage digesters have been described that spread out the digestive processes into three or more sections. See U.S. Pat. Nos. 4,604,206 and 5,637,219.
In most digesters, bacteria are added to the organic waste, and the temperature is controlled. The bacteria determine the optimum temperature for the digester to operate efficiently. Two common temperature ranges of digesters are a mesophilic temperature range (20xc2x0 C. to 45xc2x0 C.) or a thermophilic temperature range (50xc2x0 C. to 65xc2x0 C.). Methane production decreases if the optimal temperature range of the methanogenic bacteria is exceeded. See F. R. Hawkes et al., 1987. For example, a maximum volume of methane is produced by mesophilic anaerobic bacteria at a temperature of about 35xc2x0 C., and by thermophilic bacteria at a temperature of about 55xc2x0 C. Many digesters also control pH. Methanogenesis is pH dependent, with the optimal pH range from about 6 to about 8.
U.S. Pat. No. 6,254,775 describes an anaerobic digester system based on an upright vessel with internal matrices for bacteria immobilization.
U.S. Pat. No. 5,863,434 describes a process for psychrophilic (low temperature) anaerobic digestion of organic waste comprising the steps of intermittently feeding waste to a single chamber reactor containing sludge previously adapted to organic waste, and allowing the waste to react with the sludge. The waste and sludge eventually settle to form a liquid supernatant zone, which is removed as effluent, and a sludge zone.
U.S. Pat. No. 5,637,219 describes a complex, multi-stage anaerobic digester that is based on an internal rotor assembly that provides for solids mixing and for heat and mass transfer. The digester is divided by the rotor assembly into at least three or more chambers. Initially, the digester is seeded using a mixed population of anaerobic bacteria.
U.S. Pat. No. 4,885,094 describes a temperature-controlled anaerobic digester for low strength organic wastes using anaerobic microorganisms. Anaerobic digestion was accelerated by initially adding a mixture of anaerobic microorganisms, by adjusting the carbon to nitrogen ratio using waste sugar or sugar-containing product, by adjusting the nitrogen to phosphorus ratio if necessary, by controlling the pH between about 6.5 to about 8.0, and by controlling the temperature between about 30xc2x0 C. to about 50xc2x0 C. For wastes with 2 to 5% solids, the wastes were pretreated by adding an alkaline solution, heating, or pre-digesting. The main compartments was constructed with alternatively disposed baffles that produced a winding path flow through the compartment.
U.S. Pat. No. 4,604,206 describes a complex anaerobic digester with four different treatment sections to separate the acid-forming and gas-forming phases of anaerobic digestion and the mesophyllic and thermophilic bacteria. In each section is a rotating biological contractor and series of partitions to create zones in which the waste concentration is high and reaction rates are maximized. The digester has multiple internal heaters to control the temperature. The microorganisms in each section are pre-established on fixed media matrices that helps prevent microbial movement from one compartment to the next.
U.S. Pat. No. 4,246,099 describes an aerobic/anaerobic digestion process in which, prior to anaerobic digestion, the sludge is heated and oxygenated to partially decrease the biodegradable volatile suspended solids.
An unfilled need exists for a simple, inexpensive anaerobic digester that can efficiently treat organic waste of higher solids content at a shorter residence time than can conventional anaerobic digesters.
We have discovered a simple, reliable, inexpensive, and efficient anaerobic digester for treating organic wastes at a shortened residence time. The anaerobic digester is a multi-chambered digester that can handle wastewater and sludge in large volumes at a high flow rates, using a plug-flow system. The digester also allows collection of methane for use as an energy source. The reactor comprises a sequential series of reaction chambers in a design that does not mechanically stir and mix the waste as it passes through the digester. The chambers may optionally be contained within a single vessel, in a manner that promotes serpentine flow, or they may comprise separate vessels linked one to another. The volume of the chambers may be selected to control the relative residence times of the waste to select an anaerobic microorganism group or groups that can efficiently digest the waste presented to each chamber. The flow of waste is controlled to ensure that the waste passes through each chamber before exiting. Under most conditions, no deliberate addition of particular bacteria is necessary. The digester works efficiently using the microbes native to the waste material. After the reaction chambers, and just prior to the exit port for the effluent, a settling chamber is located to remove any microbes and additional solids from the effluent. In one embodiment, the reactor comprises four sequential chambers. However, other numbers of chambers and geometries will achieve similar results if the residence time in each chamber is properly adjusted. Neither pH nor temperature was controlled; however, for a higher yield of methane, pH could be controlled from about 6 to about 8.